Heat transfer surface for air preheater

ABSTRACT

A heat transfer element includes a plurality of adjacent heat transfer plates. Each of the heat transfer plates has oppositely disposed first and second heat transfer surfaces and a plurality of laterally spaced notches. A first heat transfer plate of each pair of adjacent heat transfer plates has at least one tall notch and the second heat transfer plate of each pair of plates has at least one short notch, where the notch height of the tall notch is greater than the notch height of the short notch. Each of the tall notches of a heat transfer plate are received in a short notch of an adjacent heat transfer plate to thereby define flow channels therebetween.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to heat transfer element assemblies. More particularly, the present invention relates to heat transfer element assembly adapted for use in a rotary regenerative air preheater.

[0002] Rotary regenerative air preheaters are commonly used to transfer heat from the flue gases exiting a furnace to the incoming combustion air. A typical rotary regenerative heater has a cylindrical rotor divided into compartments in which are disposed and supported spaced heat transfer plates which, as the rotor turns, are alternately exposed to a stream of heating gas and then upon rotation of the rotor to a stream of cooler air or other gaseous fluid to be heated. As the heat transfer plates are exposed to the heating gas, they absorb heat therefrom and then when exposed to the cool air or other gaseous fluid to be heated, the heat absorbed from the heating gas by the heat transfer plates is transferred to the cooler gas. Most heat exchangers of this type have their heat transfer plates closely stacked in spaced relationship to provide a plurality of passageways between adjacent plates for flowing the heat exchange fluid therebetween.

[0003] Heat transfer elements for regenerative air preheaters have several requirements. Most importantly, the heat transfer element must provide the required quantity of heat transfer or energy recovery for a given depth of the heat transfer element. Conventional heat transfer elements for preheaters use combinations of flat or ribbed form-pressed or rolled-pressed steel sheets or plates. When in combination, the plates form flow passages for the movement of the flue gas stream and air stream through the rotor of the preheater. The surface design and arrangement of the heat transfer plates provides contact between adjacent plates to define and maintain the flow passages through the heat transfer element

[0004] Due to the close proximity of the heat transfer sheets, any relative movement between the sheets will result in wear. Since the individual sheets generally have a relatively small thickness, such wear can result in the development of holes in the sheet material. Further, the wear allows for greater relative movement between the heat transfer sheets, thereby accelerating the process. In applications where the flue gasses are expected to contain highly corrosive elements, the surfaces of the heat transfer sheets are coated with a porcelain enamel material to provide greater corrosion resistance. Movement induced wear will result in localized failure of the enamel material and a loss of corrosion resistance. Conventional heat transfer elements include a perimeter structure, such as a basket, which applies an external pressure to the heat transfer surfaces to lock the heat transfer sheets together and thereby prevent relative movement therebetween. However, it is common for heat transfer elements installed in horizontal shaft air preheaters to lose the packing pressure provided by the basket structure, allowing relative movement between the heat transfer sheets during rotation of the rotor.

[0005] Heat transfer elements are subject to fouling from particulates and condensed contaminants, commonly referred to as soot, in the flue gas stream. Therefore, another important performance consideration is low susceptibility of the heat transfer elements to significant fouling, and furthermore easy cleaning of the heat transfer element when fouled. Fouling of the heat transfer elements is conventionally removed by soot blowing equipment emitting pressurized dry steam or air to remove by impact the particulates, scale and contaminants from the heat transfer elements. The heat transfer elements must therefore also survive the wear and fatigue associated with soot blowing.

SUMMARY OF THE INVENTION

[0006] Briefly stated, the invention in a preferred form is a heat transfer element which comprises a plurality of adjacent heat transfer plates. Each of the heat transfer plates has oppositely disposed first and second heat transfer surfaces and a plurality of laterally spaced notches. Each of the notches includes adjacent, mutually parallel, first and second lobes, with each first lobe extending transversely from the first heat transfer surface to a crest and each second lobe extending transversely from the second heat transfer surface to a crest. The crests of the first and second lobes define a notch height. A first heat transfer plate of each pair of adjacent heat transfer plates has at least one tall notch and the second heat transfer plate of each pair of plates has at least one short notch, where the notch height of the tall notch is greater than the notch height of the short notch. Each of the tall notches of a heat transfer plate are received in a short notch of an adjacent heat transfer plate to thereby define flow channels therebetween.

[0007] In a first embodiment of the invention, the notches of each heat transfer plate comprise at least one tall notch and at least one short notch. In a second embodiment of the invention, the notches of each first heat transfer plate of each pair of plates comprises a plurality of tall notches and the notches of each second heat transfer plate of each pair of plates comprises a plurality of short notches.

[0008] The notches of each heat transfer plate are substantially equidistantly laterally spaced apart. Each tall notch of each heat transfer plate has substantially the same notch height and each short notch of each heat transfer plate has substantially the same notch height. Each lobe has a lateral width, with the widths of the first and second lobes of each short notch being greater than the widths of the first and second lobes of each tall notch. The intermediate surface formed between adjacent notches of a heat transfer plate may include one or more protuberances extending transversely from at least one of the first and second heat transfer surfaces.

[0009] An object of the invention is to provide a heat transfer element having improved heat transfer capacity.

[0010] Another object of the invention is to provide a heat transfer element having constituent heat transfer plates which do not move relative to each other.

[0011] A still another object of the invention is to provide a heat transfer element that is easy to clean.

[0012] These and other objects of the invention will be apparent from review of the specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings in which:

[0014]FIG. 1 is a perspective view of a conventional rotary regenerative air preheater which contains heat transfer element assemblies made up of heat transfer plates.

[0015]FIG. 2 is a perspective view of a conventional heat transfer element assembly showing the heat transfer plates stacked in the assembly.

[0016]FIG. 3 is a fragmentary end-on-view of a first embodiment of a heat transfer assembly in accordance with the invention.

[0017]FIG. 4 is an exploded view of the heat transfer element of FIG. 3.

[0018]FIG. 5 is a fragmentary end-on-view of a second embodiment of a heat transfer assembly in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0019] With reference to FIG. 1, a conventional rotary regenerative preheater is generally designated by the numerical identifier 10. The air preheater 10 has a rotor 12 rotatably mounted in a housing 14. The rotor 12 is formed of diaphragms or partitions 16 extending radially from a rotor post 18 to the outer periphery of the rotor 12. The partitions 16 define compartments 20 therebetween for containing heat exchange element assemblies 22.

[0020] The housing 14 defines a flue gas inlet duct 24 and a flue gas outlet duct 26 for the flow of heated flue gases through the air preheater 10. The housing 14 further defines an air inlet duct 28 and an air outlet duct 30 for the flow of combustion air through the preheater 10. Sector plates 32 extend across the housing 14 to divide the air preheater 10 into an air sector and a flue gas sector. The arrows of FIG. 1 indicate the direction of a flue gas stream 34 and an air stream 36 through the rotor 12. The hot flue gas stream 34 entering through the flue gas inlet duct 24 transfers heat to the heat transfer element assemblies 22 mounted in the compartments 20. The heated heat transfer element assemblies 22 are then rotated to the air sector of the air preheater 10. The stored heat of the heat transfer element assemblies 22 is then transferred to the combustion air stream 36 entering through the air inlet duct 28. The cold flue gas stream 34 exits the preheater 10 through the flue gas outlet duct 26, and the heated air stream 36 exits the preheater 10 through the air outlet duct 30. FIG. 2 illustrates a typical heat transfer element assembly or basket 22 showing a general representation of heat transfer plates 38 stacked in the assembly 22.

[0021]FIG. 3 depicts a first embodiment of the invention showing portions of three stacked heat transfer plates 38. All of the heat transfer plates 38 are basically identical, composed of thin sheet metal capable of being rolled or stamped to the desired configuration. This is advantageous in that only one type of plate 38 needs to be manufactured. Each plate 38 has a series of notches 40, 42 at spaced intervals which extend longitudinally and parallel to the direction of the flow of the airstream 36 and the gas stream 34 through the rotor 12 of the air preheater 10. These notches 40, 42 maintain adjacent plates 38, 38′ a predetermined distance D apart and form the flow passages or channels 44 between the adjacent plates 38, 38′. Each notch 40, 42 comprises one lobe 46 projecting outwardly from a first surface 48 of the plate 38, 38′ on one side and another lobe 50 projecting outwardly from the second surface 52 of the plate 38, 38′ on the other side. Each lobe 46, 50 is essentially in the form of a U-shaped groove with the apexes of the grooves directed outwardly from the plate 38, 38′ in opposite directions.

[0022]FIG. 4 is an exploded view of the assembly 22 of FIG. 3. As is more readily apparent in this view, each heat transfer plate 38, 38′ has alternating short and tall notches 40, 42, where each tall notch 42 has substantially the same height Ht and each short notch 40 has substantially the same height Hs and where Ht>Hs. The pitch 58 of the notches, i.e., the distance between adjacent tall and short notches 42, 40, is substantially equal for all short/tall notch pairs such that the lobes 46, 50 of the tall notches 42 nest within the lobes 46, 50 of the short notches 40 of an adjacent plate 38, 38′ with the outer surface of the crest 60 of each tall notch 42 engaging the inner surface of the crest 60 of the short notch 40. The width Wt of the tall notches 42 is substantially equal to the width Ws of the short notches 40, with the greater slope of the tall notches 42 facilitating insertion of tall notches 42 into the short notches 40. In a preferred embodiment Ht=0.690 inches, Hs=0.322 inches, Wt=0.350 inches, and Ws=0.350 inches.

[0023] The heat transfer surface 62 intermediate the notches 40, 42 of a plate 38 is held at a distance D from the heat transfer surface 62 of the adjacent plate 38′ which is substantially equal to difference in height of the notches 40, 42, that is D=Ht−Hs. The ratio of Ht to Hs can be adjusted so the frontal area of each air flow opening is equalized for uniform air flow through the element. The heat transfer surface 62 between the notches may have protuberances to cause air turbulence in the spaces between the heat transfer sheets.

[0024]FIG. 5 depicts a second embodiment of the invention showing portions of three stacked heat transfer plates 64, 66. In this embodiment, the assembly 22′ is comprised of two types of heat transfer plates 64, 66, with all of the plates of the first type 64 being substantially identical and all of the plates of the second type 66 being substantially identical. All of the heat transfer plates 64, 66 are composed of thin sheet metal capable of being rolled or stamped to the desired configuration. Each plate 64, 66 has a series of notches 68, 70, respectively, at spaced intervals which extend longitudinally and parallel to the direction of the flow of the heat exchange fluid through the rotor 12 of the air preheater 10. Each notch 68, 70 comprises one lobe 72 projecting outwardly from a first surface 74 of the plate 64, 66 on one side and another lobe 76 projecting outwardly from the second surface 78 of the plate 64, 66 on the other side. Each lobe 72, 76 is essentially in the form of a U-shaped groove with the apexes of the grooves directed outwardly from the plate 64, 66 in opposite directions.

[0025] Each plate of the first type 64 has alternating tall notches 68 and each plate of the second type 66 has alternating short notches 70, where each tall notch 68 has substantially the same height Ht′ and each short notch 70 has substantially the same height Hs′ and where Ht′>Hs′. The pitch of the notches 68, 70, i.e., the distance between adjacent notches 68, 70, is substantially equal and is substantially the same for each type of plate 64, 66. As is apparent from FIG. 5, the lobes 72, 76 of the tall notches 68 nest within the lobes 72, 76 of the short notches 70 of an adjacent plate with the outer surface of the crest of each tall notch 68 engaging the inner surface of the crest of the short notch 70. The width Wt′ of the tall notches 68 is substantially equal to the width Ws′ of the short notches 70, with the greater slope of the tall notches 68 facilitating insertion of tall notches 68 into the short notches 70. In a preferred embodiment Ht′=0.690 inches, Hs′=0.322 inches, Wt′=0.350 inches, and Ws′=0.350 inches.

[0026] The heat transfer surface 80 intermediate the tall notches 68 is held at a distance D′ from the heat transfer surface 82 intermediate the short notches 70 which is substantially equal to difference in height of the notches, that is D′=Ht′−Hs′. The ratio of Ht′ to Hs′ can be adjusted so the frontal area of each air flow opening is equalized for uniform air flow through the element. The heat transfer surface 80, 82 between the notches 68, 70 may have protuberances to cause air turbulence in the spaces between the heat transfer plates 64, 66.

[0027] The nesting of the tall notches 42, 68 in the short notches 40, 70 form closed channels 44, 84 that facilitate cleaning of the heat transfer surface 62, 80, 82. The closed channel 44, 84 contains the energy of the sootblower or water washing jet, allowing maximum cleaning action from the energy and preventing the dissipation of energy allowed by conventional heat transfer assemblies.

[0028] It should be appreciated that the interlocking heat transfer surface design provides for longer life by preventing the relative motion between the two plates of heat transfer surface thus preventing the wearing and ultimate loosening of the heat transfer surfaces. The interlocking design also facilitates cleaning. This assembly 22, 22′ is especially suited, but not exclusively, to horizontal shaft air preheaters 10 where the heat transfer plates 38, 64, 66 of the assembly 22, 22′ are coated with enamel to prevent corrosion. The interlocking notches form a rigid block of heat transfer material that will not shift during the rotation of the rotor. The heat transfer plates 38 are easily trimmed to assure proper nesting and to form the proper overall shape for the basket.

[0029] While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation. 

What is claimed is:
 1. A heat transfer element comprising a plurality of adjacent heat transfer plates, each heat transfer plate having oppositely disposed first and second heat transfer surfaces and defining a plurality of laterally spaced notches, each of said notches comprising adjacent, mutually parallel, first and second lobes, each first lobe extending transversely from the first heat transfer surface to a crest and each second lobe extending transversely from the second heat transfer surface to a crest, the crests of the first and second lobes defining a notch height, adjacent heat transfer plates defining a pair of plates, a first heat transfer plate of each pair of plates having at least one tall notch and a second heat transfer plate of each pair of plates having at least one short notch, the notch height of the tall notch being greater than the notch height of the short notch, each of said tall notches of a heat transfer plate being received in a short notch of an adjacent heat transfer plate to thereby define flow channels therebetween.
 2. The heat transfer element of claim 1 wherein said notches of each heat transfer plate comprise at least one tall notch and at least one short notch.
 3. The heat transfer element of claim 1 wherein the notches of each of the heat transfer plates comprises a plurality of tall notches and a plurality short notches.
 4. The heat transfer element of claim 1 wherein the notches of each first heat transfer plate of each pair of plates comprises a plurality of tall notches and the notches of each second heat transfer plate of each pair of plates comprises a plurality of short notches.
 5. The heat transfer element of claim 1 wherein the notches of each heat transfer plate are substantially equidistantly laterally spaced apart.
 6. The heat transfer element of claim 1 wherein each tall notch of each heat transfer plate has substantially the same notch height and each short notch of each heat transfer plate has substantially the same notch height.
 7. The heat transfer element of claim 1 wherein each lobe has a lateral width, the widths of the first and second lobes of each short notch being greater than the widths of the first and second lobes of each tall notch.
 8. The heat transfer element of claim 1 wherein the notches of each heat transfer plate define at least one intermediate surface having a plurality of protuberances extending transversely from at least one of the first and second heat transfer surfaces.
 9. A heat transfer element comprising a plurality of adjacent heat transfer plates, each heat transfer plate having oppositely disposed first and second heat transfer surfaces and defining a plurality of laterally spaced notches, each of said notches comprising adjacent, mutually parallel, first and second lobes, each first lobe extending transversely from the first heat transfer surface to a crest and each second lobe extending transversely from the second heat transfer surface to a crest, the crests of the first and second lobes defining a notch height, said notches of each heat transfer plate comprising a plurality of alternating tall notches and short notches, each of the tall notches having a substantially uniform notch height Ht and each of the short notches having a substantially uniform notch height Hs, Ht being greater than Hs, wherein for each pair of adjacent heat transfer plates, the second lobes of the tall notches of a first heat transfer plate of the pair are received in the first lobes of the short notches of a second heat transfer plate of the pair and the second lobes of the tall notches of the second heat transfer plate are received in the first lobes of the short notches of the first heat transfer plate to thereby define flow channels therebetween.
 10. The heat transfer element of claim 9 wherein the notches of each heat transfer plate are substantially equidistantly laterally spaced apart, adjacent notches defining intermediate surfaces therebetween.
 11. The heat transfer element of claim 10 wherein at least one of the intermediate surfaces of at least one of the heat transfer plates has a plurality of protuberances extending transversely from of the heat transfer surfaces.
 12. The heat transfer element of claim 9 wherein in the flow channels, the second heat transfer surface of the first heat transfer plate is at a distance D from the first heat transfer surface of the second heat transfer plate, wherein D=Ht−Hs
 13. The heat transfer element of claim 9 wherein each lobe has a lateral width, the widths of the lobes of each short notch being greater than the widths of the lobes of each tall notch.
 14. A heat transfer element for a rotary regenerative preheater comprising a plurality of adjacent heat transfer plates, each heat transfer plate defining a plurality of alternating, laterally spaced, tall and short notches, each of said notches comprising adjacent, mutually parallel, lobes extending transversely from opposite sides of said heat transfer plate, the tall notches having a substantially uniform height Ht and width Wt and the short notches having a substantially uniform height Hs and Ws, wherein Ht>Hs and Ws>Wt and wherein for each pair of adjacent heat transfer plates, lobes of the tall notches of each heat transfer plate are received in complementary lobes of the short notches of the other heat transfer plate to thereby define flow channels therebetween.
 15. A heat transfer element comprising a plurality of adjacent heat transfer plates, each heat transfer plate having oppositely disposed first and second heat transfer surfaces and defining a plurality of laterally spaced notches, each of said notches comprising adjacent, mutually parallel, first and second lobes, each first lobe extending transversely from the first heat transfer surface to a crest and each second lobe extending transversely from the second heat transfer surface to a crest, the crests of the first and second lobes defining a notch height, adjacent heat transfer plates defining a pair of plates, a first heat transfer plate of each pair of plates having a plurality of tall notches and a second heat transfer plate of each pair of plates having a plurality of short notches, each of the tall notches having a substantially uniform notch height Ht and each of the short notches having a substantially uniform notch height Hs, Ht being greater than Hs, wherein for each pair of plates, the second lobes of the tall notches of the first heat transfer plate are received in the first lobes of the short notches of the second heat transfer plate to thereby define flow channels therebetween. 